The combustion of fuel in an excess oxygen-comprising gas mixture is an efficient way of producing energy in stationary and mobile applications. Fuel efficient diesel engines are used almost exclusively in trucks and increasingly in cars, especially in Europe. In lean mixture combustion, the emissions usually have quite a low content of carbon monoxide (CO) or hydrocarbons (HC), but, regarding oxides of nitrogen (NOx) and particles (particulate matter, PM), problems may arise in reaching the emission standards imposed by authorities. Moreover, carbon monoxide and hydrocarbon emissions can be eliminated effectively, with an oxidation catalyst, but the reduction of nitrogen oxides and particles requires the use of other types of after-treatment methods. Particles can be removed effectively with various particulate filters. The reduction of nitrogen oxides from an excess oxygen-comprising waste gas is difficult, because possible reducing agents tend to oxidize instead of reacting with nitrogen oxides.
The reduction of nitrogen oxides in incineration facilities became a topical issue in the early 1970s in Japan where NOx emission restrictions were imposed for cutting down smog that causes problems particularly in large cities. The selective catalytic reduction (SCR) of nitrogen oxides with ammonia (NH3) was developed for these objectives. In the catalyst, ammonia reacts primarily with NOx despite the presence of excess oxygen. As a matter of fact, oxygen promotes the reaction in SCR-catalysts, which, since that time, have been TiO2-based in commercial products, comprising vanadium, tungsten and molybdenum oxides as active components and stabilizers. There are also numerous publications about other types of SCR-catalysts, which are oxide-, zeolite- or carbon-based or mixtures thereof. SCR-catalysts are nowadays nearly always honeycomb type, whereby the pressure drop and clogging remain modest. The catalysts can be extruded from SCR-catalyst mass or coated on the surface of a honeycomb type carrier material. The carrier material is generally ceramic or metallic.
The main reaction of SCR in an excess oxygen-comprising mixture can be presented as follows:4NH3+4NO+O2-->4N2+6H2O  (1)
Ammonia can be introduced by way of purpose-built injection nozzles as a gas or an aqueous solution into a waste gas slightly in front of the catalyst. When ammonia is used as a reducing agent, the ammonia is immediately in its correct makeup, and the restricting factor can be mixing within the gas flow or vaporization of the aqueous solution.
It was discovered in the 1980s that ammonia can be replaced by using also other reducing agents, such as urea or cyanuric acid, with a content of ammonia derivatives or nitrogen. The SCR systems designed for automobiles have been principally based from the beginning on the use of urea as a reducing agent, since the use, storage and transport of urea and urea solution is safe as compared to ammonia. Urea (CO(NH2)2) comprises two NH2 groups and the disintegration of one urea molecule produces two molecules of ammonia in a water-comprising gas mixture. Pure urea is a solid white substance readily soluble in water in high concentrations. In urea-SCR systems for truck and power plant applications, the employed reducing agent is indeed a urea-water-solution:(NH2)2CO→NH3+HNCO  (2) thermolysisHNCO+H2O→NH3+CO2  (3) hydrolysis
Urea is carried as a 32.5 percent solution in truck applications, and the solution is introduced into the exhaust gas along with air or alone as a solution. The use of air with the solution provides a mixture that can be delivered under pressure into the hot exhaust gas. When the employed reducing agent is a urea solution, it is necessary to leave a sufficient amount of time for the urea solution to become mixed in the pipe for vaporization, as well as for the thermolysis (reaction 2) and hydrolysis (reaction 3) of urea. The urea solution must be injected to a sufficient distance from the forward edge of an SCR catalytic converter for the urea to have been reacted into ammonia consistently in radial direction. In truck applications (engine displacement 4-20 liters), the amounts of exhaust gas are so large that there is generally needed a circular cell 250-400 mm in diameter for maintaining the linear speeds and pressure drop within a regulatory range and for enabling the SCR-catalyst to function without upsetting the engine operation. Therefore, the mixing of urea in radial direction is important. The thermally occurring thermolysis and hydrolysis require a sufficient amount of time, which is why the urea introduction point may be as far as a few meters away from a forward edge of the SCR-catalyst.
Trucks may also involve the use of a diesel oxidation catalyst to promote the oxidation of hydrocarbons, carbon monoxide and NO into NO2. Oxidation catalysts normally employ highly sulfur-resistant platinum (Pt) as an active metal. In view of heat utilization, it is desirable to install the catalysts as close to the engine as possible. The oxidation catalyst is beneficial for the operation of an SCR-catalyst, because the removal of HC and the resulting NO2 provide a remarkable promotion of SCR reactions. It has been proposed that a special hydrolysis catalyst (H-catalyst) be used in front of the SCR-catalyst to promote the mixing of urea and the hydrolysis at various temperatures (Döring and Jacob, 21st Vienna Motor Symposium 2000). At the same time, it was proposed that the hydrolysis catalyst and the pre-oxidation catalyst be fitted side by side, whereby urea is only introduced into a side flow of exhaust gas. Hence, it is possible that the automobile having an SCR system can be fitted with a pre-oxidation catalyst, a hydrolysis catalyst, an SCR-catalyst, and a re-oxidation catalyst, the purpose of the latter being to remove the possible ammonia left in the exhaust gas after the SCR reaction (EP0896831). The mixer type assembly has also been referred to as a vaporizer with a catalytic coating on its surface. Another argument mentioned for the use of an H-catalyst is that the SCR-catalyst volume can be reduced e.g. by 10-30% (EP0555746).
The use of a hydrolysis catalyst has been proposed either alone or in combination downstream of a separate vaporizer element (EP 0487886). Mentioned as catalyst coatings are TiO2, Al-oxide, SiO2 or a mixture thereof, which may also be accompanied by SO3 or WO3, i.a. for acidic properties or thermal stabilization. The specific surface area has been said to exceed 10 m2/g (EP 0487886). It has also been mentioned that, in addition to these, the hydrolysis catalyst comprises zeolite (H-mordenite, H-ZSM5) (EP 0555746). The H-catalyst must have a resulting ammonia decomposition activity as slight as possible (EP 0487886), as otherwise extra loss is generated in urea consumption.
The hydrolysis of urea and its mixing with a gas flow can be promoted by improving the actual injection of urea, which can be assisted by using various nozzles, supply pressure adjustment and control engineering. An aspect of major importance is how far and at which point in the apparatus the injection of urea is conducted. The aspect that must be considered in terms of dimensions is to design the urea spray and flow channel in such a shape that there is no spraying of urea onto cold walls. Should urea end up as a droplet on a wall, or on a cold wall at that, there is a hazard of generating undesirable by-products, thus increasing the loss of urea. In SCR, the operational efficiency of urea in the reduction of nitrogen oxides must be more than 90%, because, in a standard European test cycle, the consumption of urea amounts for example to 3-6% of the consumption of diesel fuel, thus representing a major expense.
A proposal has also been made for the introduction of solid urea as a powder, thus avoiding the need to carry water along with a solution. The system may also include a hydrolysis catalyst just like in the introduction of a liquid urea solution (EP 0615777). A predicament in these systems is often the consistent dosage of powder into exhaust gas in various conditions.
Problems in the described hydrolysis catalysts may include the fact that, with a single H-catalyst, it is very difficult to achieve simultaneously an effective low temperature hydrolysis, mixing, and slight decomposition of NH3 into nitrogen or oxides of nitrogen over the entire required temperature range (100-600° C.). The composition and dimensional design of an H-catalyst, which is effective in hydrolysis at 150-200° C., is often too active at high temperatures and NH3 decomposes prior to the SCR reaction. The hydrolysis catalysts have been described to be high temperature catalysts with a large surface area (10 m2/g) and plenty of porosity, and particularly small pores. It has also been described that the H-catalyst has specifically involved the use of compounds that provide surface acidity for the adsorption of NH3. In this case, the dwell time for ammonia becomes nevertheless longer as a result of adsorption, pores, and volume, and is the longest at low temperatures, whereby kinetically NH3 has more chances to decompose and stay for too long in the H-catalyst. The H-catalysts have been described to be mixer structures, wherein the mixing has been described to primarily occur within a single channel of the cell and the aperture numbers have been about 150 cpsi (cells per in2) and the amount of coating to be reasonably high, i.e. about 150-200 g/L (EP 0896831). Some mixing inside a cell channel is achieved with various flow barriers and claws, but mixing in the reactor's radial direction remains insignificant, whereby the inconsistency in the radial direction of flow and particularly in temperature may even be accentuated. Such structures resemble catalyst honeycomb cells, which, when compared to an empty exhaust pipe, have a large geometric surface area (GSA) and amount of catalyst, a low Reynolds number (→mass transfer efficiency) in channels, weak mixing in radial direction. Such structures are good in terms of promoting a catalytic reaction, but the urea and the solution must vaporize and become mixed within the gas flow before any advantage is obtained by the catalytic promotion of hydrolysis. What is desired at the same time is nevertheless good mixing and urea injection as close as possible to the SCR-catalyst or the H-catalyst's face surface. If the H-catalyst has an excessively high aperture number, there is a hazard of the urea-water spray striking the face surface of a dense cell with negative consequences similar to those resulting from the spray hitting the walls of a pipe. A further hazard in such a case is that the H-catalyst cell's frontal surface and the coating thereon are worn down mechanically by droplets or the dense cell is clogged by solid by-products. A common problem regarding separate hydrolysis and SCR reactors is how to convert urea completely into ammonia, how to convey the resulting NH3 into an SCR reactor for reducing nitrogen oxides with ammonia.
If the hydrolysis catalyst is installed in such a way that just part of the flow passes therethrough, it will be difficult to regulate the flow rate through the hydrolysis catalyst with a suitably sparse or dense cell so as to provide at the same time an appropriate space for the urea spray in front of the cell and to set the linear speed within a suitable range. The back pressure of a sparse cell is lower and too much flow passes through the cell. In the case of a dense cell, the back pressure is too high with the flow rate remaining low, nor can urea be injected at a site too close to the forward edge of the cell. For these reasons, the hydrolysis reactor needed further development.
Another proposal has been made for 3D mixer structures, which have been used in an uncoated condition or have been coated with a typical SCR-catalyst (static mixers). In that case, the mixing is most effective with large channel sizes, which is good for power plant applications involving large amounts of particles. Thus, the mass transfer and the distribution of urea/ammonia are consistent, yet there are problems, including e.g. a small amount of catalyst material on the walls of traditional, large-channeled static mixers, and the hydrolysis of urea is based on reactions which occur thermally or in an SCR-catalyst.